Method of making cores for cathodes of vacuum tubes



Patented Dec. 8, 1925.

UNITED .s'ra'rs PATENT OFFNZE.

HOWARD T. REEVE, OF EAST ORANGE, NEW JERSEY, ASSIGNOR TO WESTERN ELEC- TRIO COMPANY, INCORPORATED, OF NEW YORK, N. Y., A CORPORATION OF NEW YORK.

HET HOD OF MAKING CORES FOR CATHODES 0F VACUUM TUBES.

No Drawing.

To all whom it may concern:

Be it known that I, HOWARD T. REEVE, a subject of Great Britain, residin at East Orange, in the county of Essex and State of New Jersey, have invented certain new and useful Improvements in Methods of Making Cores for Cathodes of Vacuum Tubes, of which the following is a full, clear, concise, and exact description.

This invention relates to a method of making cores for electron-emitting electrodes for use in vacuum tubes, such as audions, and for convenience, will be described in connection with coated, electron-emitting cathodes of the kind disclosed in the application of J. E. Harris, Serial No. 416,386,

. thermionic cathodes and methods of making the same, filed October 12, 1920, assigned to the assignee of this application. Such cathodes are preferably of filamentary form and consist of a core and a thermionically active coating therefor, the core containing arefractory, malleable metal of low volatility, such as platinum or palladium, and a metal, such as nickelor cobalt. Nickel and cobalt, for example, are characterized by the fact that upon being baked with an oxide of an alkaline earth metal in air, they react with oxygen and ,the oxide much more readily than does platinum, to form a compound which breaks down to the original constituents when the compound is heated in a Vacuum. "After the cathode filament having the platinum nickel core and a coating therefor comprising oxides of alkaline earth metals has had its normal heating current passed through it for about sixteen hours, in the vacuum tube, it has an electron-emitting activity considerably greater than that of a filament made by coating a platinum. core with said oxides.

The object of this invention is to provide a process by which the metals of which the core is to be made, can be alloyed and then worked into wire or filamentary form, free from objectionable cracks, and without undue volatilization of the nickel or its equivalent.

The particular core, composition of mat- Application ,filed October 12, 1920. Serial No. 416,464.

ter used for the core, and method of making a filament from the core by coating the core with one or more of the oxides of the alkaline earth metals, as well as the filament consisting of the core and the coat, all of which are hereinafter described in connection with this invention, are the invention of J. E. Harris and are claimed in his application referred to above.

The process of making the core will now be set forth in detail.

The constituents of which the alloy is to .consist are thoroughly mixed in the form of finely powdered metals in the desired proportions, for example, 95% platinum and 5% nickel. The mixture is pressed in a steel an inert gas. This fusing or sintering may be done by putting the ingot in a small refractory crucible of alundum, magnesia, or the like, the crucible being placed in a vacuum furnace of any suitable type. A type which has been used with good results has an evacuated space connected with any good oil pump capable of producing a vacuum of .001 mm. of mercury, this space containing a helical heating coil of graphite, tungsten, molybdenum, or the like, which surrounds the crucible. The coil is heated by the passage of a large, low voltage electric current from the secondary winding of a step-down transformer. The. vacuum chamber. Preferably has a transparent-top so that the temperature of the ingot may be observed with an optical pyrometer. The melting point of nickel is about 1450 deees centigrade and that of platinum is about 1750 degrees centigrade, but it is not necessary to heat the metals to their melting point'in order to sinterthem or weld and alloy them sufiiciently. The temperature used for fusing or sintering the ingot is in the neighborhood of 1500 to 1800 degrees centigrade. The duration of the fusing or sintering operation should be as short as possible, in order that the amount of nickel vaporized shall be as small as possible. The duration will depend upon the size of the ingot. With a 25 gram ingot in the form of a cylinder of an inch in diameter, fusing or sintering' in the crucible would be for only a few minutes.

Another way in which the sintering temperature may be obtained is by clam in the ingot between two electrodes in a el -jar, exhausting the air from the jar, and passing. a heating current through the ingot, increasing the current until a sintering temperature of about 1500 degrees centigrade, as observed with an optical pyrometer for example, is reached, and then cutting the cur-- rent off; Where the ingot is heated in this manner, the duration of the sintering will be only momentary and will be only a quarter or a tenth as long as where the ingot is fused or sintered in a crucible in a furnace as referred to above.. Y

The pressures maintained by the pump during the fusing or .sintering have been from .01 to .001 of a millimeter of mercury, though probably a vacuum of only a few millimeters. would suffice. However, the higher the vacuum, the better will be the results, since the sintering in a vacuum or in an inert gas prevents oxidation of thenickel. Of course,.as is indicated above, the fusing or sintering maybe done in an induction fit) furnace or in any suitable vacuum furnace. When the ingot has been fused or sintered, it is cold forged to get it into the form of a rod and then the rod is swaged into wire, which is drawn through diamond dies, or thelike, to the required size, all of this forging, swaging and drawing being cold working. Hot working is not advisable for if the platinum-nickel alloy were Worked hot the nickel would be oxidized and as a consequence the material, or work, would become very brittle. The material is annealed at frequent intervals during all of this forging, swaging and drawing. Great care must be taken that the working at any stage is not pushed too far, without annealing, else the material, or work, is likely to become so brittle that it will crack. The method of annealing depends on the stage of the working-that is, on the dimensions, especially the length, of the Work. In general, the larger the diameter, the higher the annealing temperature. The temperature should be between about 1100 C. and the fusing temperature, which is in the neighborhood of 1700 C. Regarding the duration of the heating for annealing, it is only necessary to bring the work up to the annealing temperature and then let it cool off. The method of heating for annealing should preferably be such as will guard against oxidation of the nickel. When the working is in the first stages, for instance when the work is in the form ofan ingot of the size referred to above, the work may Well be heated to the annealing temperature, in the oXy-hydrogen flame, and preferably cooled in a reducing hydrogen flame (the supply of gas from the oxygen tank being shut off) until the ingot is below red heat when it is withdrawn from the flame and quenched in water. When the work has increased considerably in length, the .method preferred consists in clamping the work between electrodes, in a vacuum, and passing an'electric current through it, to give the required temperature. In any case, if the work becomes oxidized, the oxide can be broken down by heating the work in a vacuum for a few minutes at a temperature of about 1000 C.

The wire made from the ingot as described above, is suitable for use as a core for coated cathodes of the type referred to above. However, the wire is preferably cold rolled into ribbon form having, for example, a width of .3 of a millimeter and a thickness of .05 of a millimeter and the ribbon coiled and pulled out into the twisted filamentary form common for thermionic cathodes, and

shown, for example, in United States patent to Nicolson and Hull, No. 1,209,324, December 19, 1916, electron-emitting cathode and the process of manufacturing the same.

A filament having high activity can be made by coating the core described above with an oxide of any alkaline earth metal by applying a coating of the carbonate or hydroxide of the metal to the core, heating the coated core in air for about 5 to 15 minutes at about 1200 0., by passing cur rent through it, and later heating it in a vacuum for several minutes at about 1000 C., by passing a current through it, during the pumping of the vacuum tube. How ever, a more durable filament, and one having uniformity of electron-emitting activity over a longer period, is made by applying coating of strontium carbonate and barium carbonate to the platinum-nickel core by a method somewhat similar to that disclosed for applying such coatings to a platinum core in the application of Carl D. -Hocker, Serial No. 252,689, filed September 5, 1918, process of manufacturing electron-emitting cathodes, assigned to the assignee of this application. Preferably a total of about 16 coats of the carbonates should be applied. The coating material is embodied or suspended in a solid carrier which may be of waxy material such as paraffin, by stirring together the desired quantity of strontium carbonate and barium carbonate into the same batch of molten paraffin, until the mixture has cooled and then forming the mixture into rods or pieces of other convenient shape. the ingredients are 2% grams of barium carbonate, 7 grams of strontium carbonate, and 100 grams of paraffin. To apply each coat, the solid rod containing the carbonates is passed along the length of the core, the core being heated in the usual way (that is y passing a current through it) to a temerature sufliciently high to melt the parafiin, so that the coating material will be deposited on the core. After the application of each coating, current should be passed through the strip to flash it momentarily to a red heat which serves to burn off the paraffin, reducing the alkaline earth compound to an oxide which adheres closely to the filament. During the coating process, the filament may be held by any suitable supports.

After the last coat has been applied to the core, the filament is baked in air for about 5 to 20 minutes at a temperature of about 1200 C.- The filament is, of course, later heated in a vacuum for several minutes at a temperature of about 1000 C., during the pumping of the vacuum tube. After the filament has had its normal operating current passed through it for about sixteen hours, in the vacuum tube, it has an electronemitting activity about twice as great as that of a filament made by similarly coating a platinum core.

When the platinum-nickel core coated with'a carbonate of an alkaline earth metal is baked in air (as'described above), nickel oxide is formed, and an oxide of the alkaline earth metal is formed, and then a compound containing nickel and the oxide of the alkaline earth metal is formed. In the sub sequent baking operation (as described above) during the pumping out of the vacuum tube the compound breaks down, yieldof the' ng oxygen and nickel and the oxide alkaline earth metal, so that the coating then left on the core comprises the alkaline earth oxide in intimate contact with finely divided nickel.

A core com osition of approximately 95% platinum'and 5% nickel, has been found most satisfactory when due consideration is given to the malleability desirable in order that the alloy may be worked into wire, the

degree of activity of the finished cathode,

and the burn out life and especially the activity life of the cathode.

However, satisfactory cores have been made by the process of this invention with the proportion of platinum varying widely from 95%. Probably the low limit of the platinum content and also of the nickel content for a satisfactory core, is determined principally by the rate of volatiliz ation or burning out of" the cathode when in use in vacuum tubes. Pure platinum burns out more rapidly than Preferably, the relative amounts of core is increased to platinum alloyed with a small proportion of nickel; but nickel is very volatile, and too much nickel alloyed with the platinum will decrease the burn out life of the cathode. When the nickel content of the more than about 15%, the rate of burning out of the cathode becomes higher than that of a platinum core filament; and when the nickel content of the core is decreased to below about 2%, the rate of burning out of the cathode becomes of the order of that of a platinum core filament. Moreover, a nickel content of about 2% (or more) of the core material is desirable in order to make the coating material adhere strongly to the core. This invention embraces the method of making the core from constituents such as are referred to above in any proportions which give a core, having a low volatilit a (1 characterized by the property of yield ng when coated with oxide of an alkaline e th metal and baked in air and then in a vacuum (all as described above), a prod ct having an electron-emitting. activity co siderably greater than that of a .product ads by similarly treating a platinum core.

In regard to the construction of the claims, it should be noted that fusing the ingot would include the sintering step set forth in the claims. I

The nickelin the platinum-nickel core produces a desirable increase in the ohmic resistance of the core, as do the iridium and rhodium in the platinum cores now in common use containing platinum mixed with varying amounts of lridium and rhodium. However, the. irdium and rhodium may. of course, also be incorporated in the mixture of powdered" platinum and nickel used in the process of this invention, for further increasing the resistance, without departing from the invention.

It should be understood that where only platinum and only nickel are referred to above, such. references are to be regarded as also including the equivalents of the platinum or of the nickel, as indicated above.

What is claimed is:

1 The method of making a core for an electron-emitting cathode which. comprises mixing metals, including platinum and nickel in powdered form, the proportion of platinum to nickel being that desired in the core, pressing the mixture into a coherent ingot, heating the ingot to a sintering tem perature in the absence of oxidizing conditions, and cold working the ingot into filamentary form with annealing of the work at high temperature, at intervals, the early stages of the cooling being done in the absence of oxidizing conditions.

2. The method" of making a core for an electron emitting cathode which comprises mixing platinum and a metal of the nickelcobalt group in proper proportions, forming a coherent ingot thereof, heating said ingot to a. sintering temperature in the absence of oxygen, cold working the ingot in the filamentary form, and annealing the work at high temperature at intervals, the

early stages of the cooling being done in the absence of oxidizing. conditions.

In witness whereof, I hereunto subscribe 10 my name this 10th day of September A. 1)., 1920.

HOWARD 'r. REEVE. 

